As one type of a stepping motor, a PM type (Permanent Magnet Type) stepping motor is widely known. The PM type stepping motor is one in which a magnet (permanent magnet) with magnetic poles placed in a circumferential direction is placed on an outer periphery of a rotor, and a stator including a driving coil is disposed around the magnet and, in general, stators of two phases are disposed to be aligned in a motor axial direction.
FIG. 3 shows a conventional PM type stepping motor of this kind seen in Japanese Patent Laid-Open No. 9-289152 described below.
The stepping motor is constructed roughly by a cylindrical rotor 52 integrally provided with a motor shaft 51 in a center, a stator unit 53 placed at an outer periphery of the rotor 52 via a predetermined gap, and a motor case 56 which rotatably supports the motor shaft 51 and the rotor 52 via bearings 54 and 55, and houses the rotor 52 and the stator unit 53 inside.
Here, the rotor 52 includes a multipolar magnet which is magnetized in a circumferential direction on its outer peripheral portion. The stator unit 53 includes stators 53A and 53B of two phases that are a phase A and a phase B, the stators 53A and 53B are constructed by pairs of yokes 61 having the number of poles corresponding to the number of magnetic poles of the magnet, coils 62A and 62B through which a controlled electric current passes, and resin bobbins 63. By switching the direction of the electric current which is passed through the coils 62A and 62B of the respective phases, polarity of the poles is changed, so that the rotor 52 including the magnet is synchronously rotated.
Incidentally, in the stepping motor of this type, a rotor 70 as shown in FIG. 4 is generally used. The rotor 70 is formed by integrally molding a rotor body 71 made of a synthetic resin on an outer periphery of a motor shaft 75, then by integrally molding a portion to be a magnet 72 on an outer periphery of the rotor body 71, after which, predetermined magnetization is performed to make the magnet 72, and the magnet 72 is formed into a cylindrical shape having a uniform wall thickness entirely. The main reason of adopting such a structure is to reduce the entire weight of the rotor 70 and reduce the production cost by reducing the use of the magnet material which is high in specific gravity and expensive.
However, in the above described conventional stepping motor, there is the problem that the magnetic force becomes small and desired driving torque cannot be obtained if the wall thickness is made small, when the entire magnet 72 is constructed to be of the same wall thickness as described above, though reduction in weight is achieved, and on the other hand, when the wall thickness is made large, the weight of the magnet 72 becomes heavy, as a result of which, there arises the problem that drive responsiveness becomes low due to increase in inertia mass of the rotor 70.